That latest upgrade to your boat's instruments or electrical system may leave you sitting in the middle of the lake, "dead in the water," because you didn't understand your boat's wiring. While each boat's wiring may vary slightly, the basic principles of boat wiring are the same. With a bit of effort, you can understand these principles, and with a bit more effort, you'll become a safer boater because your boat's wiring doesn't have to remain a mystery.
Look at your boat's wiring diagrams, found in your owner's manual. You'll notice that, ultimately, the ground wires for everything are connected to the boat's common ground. The power leads for everything are connected to fuses before being connected to the common power buss--a block used to consolidate the positive connections in a single location that has a single lead to the battery.
Compare the wiring diagrams with what can be seen on the boat. Instruments--and other electrical or electronic bits and pieces--may be "daisy chained" together. The instrument light or power wire for one connects to the instrument light or power wire of another and another, before the last instrument connects to the common power buss, which is connected to the battery.
Write to your boat's builder or call your dealer if you have questions. The wiring diagram may show "Connection to starter" without showing how the starter is wired. Manufacturers' wiring diagrams are complete for the boat's wiring, but don't address the wiring of the engine or outboard motor.
You can learn quite a bit about electricity and safe boat wiring by taking a course from the U.S. Power Squadrons, a non-profit, educational and community service boating organisation. Courses on Marine Electronics, which include basic electricity and boat wiring are available online and may be available in your area.